Hello - I know you've had loads of great advice, and you've seen the paediatrician and everything, but in an earlier post you said you might be the latching-on that's not quite right and it might be worth listening to that instinct.
It reminded me that I was having similar problems at around the same age, and I went back to the books and 're-learnt' the latch. It helped no end!
How does he go on? My daughter had stopped opening her mouth wide and was going onto the nipple with a little nibbly kind of action - like sucking up spaghetti. I though it was very cute and delicate, but in fact was the root of all her discomfort...
Anyway, probaby useless but I thought it might be worth a mention!
Good luck, hope things continue to improve, am so glad he's gaining weight
and is a happy little bean generally xx